Organizers called the rally the largest anti-tar sands protest in the Northeast. They said it demonstrated widespread opposition to potential plans by oil companies to run tar sands oil through a 236-mile pipeline running from Montreal to South Portland.

Tar sands oil, more properly known as bituminous sands, is a type of crude oil deposit – a sludgy mixture of sand, clay and petroleum. The material is more difficult to transport than conventional crude oil and critics say its acidity makes it more likely to eat through pipes and cause spills.

Critics of the pipeline have pointed to existing safety concerns in their efforts to block any use of it for tar sands. The 72-year-old pipeline, which carries crude oil from ships docked in South Portland to refineries in Canada, runs near Sebago Lake, the primary water supply for the greater Portland area.

But industry representatives reject claims by critics that tar sands oil is more corrosive.

"There is not a single scientific, peer-reviewed study out there showing tar sands is any more corrosive than other heavy crude oils that have passed safely through pipelines in this country for decades," John Quinn, executive director of the New England Petroleum Council, told a Portland City Council hearing last week during discussion about a resolution to ban the city's use of tar-sands oil.

Protesters filled Monument Square Saturday morning before marching to the Maine State Pier. Members of participating organizations spoke briefly before the march, as drums beat out a steady rhythm. A small marching band played along the route as the crowd marched to the waterfront to hear speeches of support from organizers and local politicians, including U.S. Rep. Chellie Pingree, D-Maine, and Portland Mayor Michael Brennan.

At the pier, Pingree told the crowd she would lobby the Obama administration to conduct a full environental review of moving tar sands oil through the pipeline.

"Reversing the flow of the Portland Pipeline so tar sands oil can be delivered to Portland Harbor would pose some serious environmental risks," Pingree said.

Many of the protesters came from outside the state to rally in solidarity with Maine activists. Marla Malcum of the environmental organization 350 said the group brought more than 100 people.

"We know this is a regional fight, a national fight, and we know that when we're fighting down in Massachusetts, people from Maine will come down and help us there," she said.

Jean Knight, 73, also travelled to the rally Saturday, driving more than five hours from Burlington, Vt.

At City Hall last week, Brennan and City Councilor David Marshall spoke at a press conference opposing the piping of tar sands.

“As Maine’s largest city, we have a responsibility to lead and demonstrate to others that by enacting reasonable policies we can make a real difference in the effort to halt climate change and reduce greenhouse emissions," Brennan said.

The companies involved with the pipeline say there aren't even plans at the moment to use it for tar sands. And critics of the proposed ban say it could hurt business and be difficult to enforce.

Jamie Pye, president of the Maine Energy Marketers Association said the ban would be impractical because tar sands cannot be easily identified and separated from other forms of petroleum.

"The city cannot ban what it ban what it cannot define," he said.

Ultimately, the council voted neither for nor against the ban. Instead, it sent the proposal back to its Transportation, Sustainability and Energy Committee for further review.